This invention relates to a position detector utilizing a magnetostrictive effect, and more particularly to a position detector for detecting the position of an elongated movable object which provides high detection accuracy even in the atmosphere in which a temperature change occurs.
In a prior position detector, a core wire is disposed inside a waveguide made of a ferromagnetic material and the movable object is provided with a permanent magnet at a predetermined position thereof. When a pulse current is applied from a pulse generator to the core wire, ultrasonic wave is generated by a magnetostrictive effect at the location where the permanent magnet of the movable object exists, and propagates through the waveguide.
If the permanent magnet exists at a position having a distance x from some point on the waveguide, a voltage can be detected in a coil at the time t (sec) after the application of the pulse current by the inverse magnetostrictive effect. The position x can be determined in accordance with the formula below by measuring this time t: EQU x=v.multidot.t
where v is the propagation speed of the ultrasonic wave inside the waveguide.
In conventional position detectors of the kind described above, the propagation time t of the ultrasonic wave is measured in accordance with the formula above. Therefore, if the propagation speed v changes for some reason or other, the time t also changes. Thus, the change of v directly turns out to be the detection error.